All-wheel-drive motor vehicles are the subject of continuous development, inter alia on account of the increased fuel consumption and the increasing requirements in respect of exhaust gas regulations. In this context, in particular, the at least partial electrification of the motor vehicle or the motor vehicle drive train is paramount;
the at least partial hybridization of the motor vehicle or the motor vehicle drive establishes the balancing act between a reduced fuel consumption, a sufficient range and satisfactory performance.
In the case of an all-wheel-drive motor vehicle with a transverse main drive unit which lies at the front, namely an internal combustion engine, for example, an all-wheel-drive system is required which makes a variable all-wheel-drive function possible. Here, the primary axle, that is to say the permanently driven axle, is the front axle, and the secondary axle, that is to say the axle which is not driven permanently, is the rear axle. The rear axle should be capable of being switched in in a fully variable manner according to requirements with consideration of the requirement for a variable all-wheel-drive function, and should offer the possibility of transmitting the entire available torque in a variable manner via the secondary axle, namely the rear axle. At the same time, functions such as brake force recuperation, boosting, that is to say accelerating the vehicle by way of positive drive torques both from the internal combustion engine and from an additional drive unit, such as an electric machine, and purely electric driving are possibly to be made possible. Thus, for example, an electric rear axle in addition to the transversely installed front main drive unit with a permanent front axle drive makes recuperation, boosting and purely electric driving possible. Furthermore, the electrification of the secondary axle (the rear axle in the example which is mentioned) makes it possible to dispense with a conventional all-wheel-drive clutch, in order to transmit the drive torque in a variable manner to the secondary axle.